Hello Antomatoe,
You can upload up to 20 attachments per post, in 5 at a time increments. When your in the manage attachments section. On the right there are 5 boxes that say browse next to it. I'm sure you've got this far because you have already uploaded pics. Once you have clicked browse and navigated to the file you want for each of the 5 boxes (or as many as you want), to the right of the fifth one click the upload box. Then wait until all 5 of those boxes say "no file selected" again. You can do this 4 times for a total of 20 pictures.

When you have finished uploading what you want, and the browse boxes are empty or full, just close the manage attachments window by either clicking on the red x in the upper right corner, or scroll down to the bottom and click the close window button. After you have uploaded the images you want, and have closed the manage attachments window, click the "Preview Post" button and the attachments will show under the text box you type in.

If you decide to delete a attachment, just open the manage attachments window and scroll down to view all the attachment files you uploaded. Just click the "Remove" button next to the file you want to delete.